1. Field of the Invention
The present invention generally relates to an amplifying apparatus with a differential amplifier stage located at the input stage. More particularly, this invention is directed to an amplifying apparatus suitable for a low frequency signal, which reduces shock noise that occurs when a power supply is switched on or off.
2. Description of the Related Art
For instance, an amplifying apparatus for amplifying a sound signal should meet severe requirements against shock noise that is generated in accordance with the ON/OFF action of the power supply. Particularly, regarding a power amplifying apparatus for driving loud speakers according to an output signal from the mentioned amplifier apparatus, there is a demand for an ability to suppress undesirable shock noise which i generated from loudspeakers when the power supply is switched on or off.
There is known means for suppressing such shock noise; this means uses a decoupling capacitor or ripple filter capacitor connected to a power circuit, and utilizes a time constant set by a circuit including such a capacitor to mute a sound signal output.
FIG. 3 exemplifies a conventional amplifying apparatus which suppresses shock noise caused according to the ON/OFF action of a power supply. A capacitor 11 and resistors 12, 13 and 14 constitute a ripple filter circuit. A power supply voltage V.sub.CC is divided by the resistors 12 and 13, connected in series, and the divided voltage is supplied via the resistor 14 to the capacitor 11.
The voltage V.sub.CC is further divided by resistors 15 and 16, and the divided voltage is supplied to the bases of transistors 17 and 18 whose emitters are supplied via the resistors 12 and 14 with the voltage V.sub.CC. The ratio of the resistances of the resistors 15 and 16 is so set that the base potential of the transistors 17 and 18 is equal to or higher than their emitter potential. That is, the transistors 17 and 18 are set off in the normal operational state.
The transistor 17 thus controlled has its collector connected to the base of a transistor 19, which is connected in parallel to the capacitor 11, so that the transistor 17 when on sets the transistor 19 on to thereby discharge the capacitor 11. A signal from the collector of the transistor 18 is supplied to the base of the transistor 20, so that an output signal is output in the normal state where the former transistor 18 is rendered on.
In other words, in the normal operational state where the power supply voltage V.sub.CC is set, the transistor 17 is rendered off, so is the transistor 19, thus permitting the capacitor 11 to be charged by the voltage V.sub.CC via the resistors 12 and 14. As the transistor 18 is off, the transistor 20 is rendered off and an output signal is properly output.
When the power supply is switched off in such a state, the voltage V.sub.CC falls and the base potential of the transistors 17 and 18 becomes lower than their emitter potential before the voltage of the ripple filter circuit falls. This renders the transistors 17 and 18 operative, which turns on the transistors 19 and 20. The transistor 19 being on permits the capacitor 11 to be discharged. At the same time, the output signal is led to the ground by the transistor 20 to cut off the output, thereby preventing shock noise from being generated by the power off of the power supply.
The use of the above-described circuit is effective in suppressing shock noise which is generated in accordance with the OFF operation of a single power supply, but cannot cope with a case where the ON/OFF operation of the power supply is sequentially repeated. When such an amplifier circuit is constituted by an integrated circuit, a differential amplifier circuit is provided at the input stage though not shown in FIG. 3. Such a differential amplifier circuit often has different time constants on one input side which is supplied with an input signal and the other input side which is supplied with an output signal as a negative feedback signal and this circuit is in an unbalanced state. When the power supply is switched off, switched on again, then switched off again, therefore, the unbalanced state of this differential circuit would directly appear as a variation in the output and would be output as shock noise.
For instance, in an amplifier circuit provided in a stereo system for an automobile, when a starter motor is operated to start the engine through operation of an ignition key, the power for the stereo system is cut off for a very short period of time, in which case, particularly, the generation of the aforementioned shock noise should be considered.